I'm sure I'll find 2-3 days insufficient. A colonial town on the ocean will provide decent sunrise as well as sunset photographs silhouetting various objects. There are many cobblestone streets and old buildings that i would not photograph during midday harsh lighting. See my gallery (signature). i enjoy roaming and photographing small old world details - see my Rome, Moroccan and Guanajuato galleries. Colonia will be a treasure trove for me. Did you make it to the abandoned bull ring a few kms east of Colonia? there's horse race track there as well. I could easily fill 3 days there shooting only in the best light early/late day. Montevideo looks nice and a great photo-op on it's own but given my 17 day restriction it offers little different from Buenos Aires IMO. I like to get in a variety of photographs when I travel - Buenos Aires will be architecture and people candids. Iguazu will be nature/landscape. Colonia will be old world charm and beach sunsets.